The colon makes up the longest part of the large intestine. It begins from the caecum at the ileocecal valve and ends in the rectum. The colon is about 1.5 meters long and frames the convolute of the small intestine in the abdominal cavity. However it can be shortened and lie quite flexibly in case of an incomplete rotation of the umbilical loop during embryogenesis.
The colon may be subdivided into four parts: ascending, transverse, descending and sigmoid colon. The ascending colon lies secondary retroperitoneally at the right abdominal wall and moves towards the right colic flexure at the bottom side of the liver. From there the transverse colon runs intraperitoneally towards the spleen forming the left colic flexure. This part is attached to the posterior abdominal wall by the mesocolon and is therefore very flexible. Beginning at the left colic flexure the descending colon proceeds downwards secondary retroperitoneally at the left abdominal wall and changes over to the S-shaped sigmoid colon in the left iliac fossa. As the sigmoid colon lies intraperitoneally it has a mesocolon as well. This last part of the colon ends in the rectum at the height of S2-S3.
Macroscopically the colon has some distinct morphological features compared to the small intestine. Semilunar folds arise in the inner surface through muscle contractions. These are merely caused functionally and therefore movable. These folds form pouches on the external surface (haustra). The longitudinal musculature is concentrated in three strong ribbon-like strips (taeniae coli). The mesolon is attached to the mesocolic taenia and the greater omentum to the omental taenia whereas the free (or liberal) taenia is unbound and fully visible. Another characteristic feature of the colon is the small sacculations filled with fat formed by the serosa (appendices epiploicae).
Branches of the superior mesenteric artery (right colic artery, middle colic artery and colic branch of ileocolic artery) supply the ascending and transverse colon. The descending and sigmoid colon are supplied by branches of the inferior mesenteric artery (left colic artery and sigmoid arteries). The middle and left colic arteries form the (inconstant) anastomosis of Riolan. The venous blood drains through the correspondent veins into the superior and inferior mesenteric veins. Until the Cannon-Boehm point at the left colic flexure the sympathetic innervation is carried by nerves of the superior mesenteric plexus, the parasympathetic innervation through the vagus nerve (cranial nerve X). After that point nerves of the inferior mesenteric plexus carry the sympathetic innervation, the pelvic splanchnic nerves the parasympathetic innervation.
The colon has the typical histological structure as the digestive tube: mucosa, submucosa, muscularis and serosa/adventitia. The mucosa is lined by simple columnar enterocytes (lamina epithelialis) with long microvilli. It is covered by a layer of mucus which aids the transport of the feces. The mucosa does not contain villi but many crypts of Lieberkuhn in which numerous goblet cells and enteroendocrine cells are found. The connective tissue layer (lamina propriae mucosae) is filled with macrophages, plasma cells and other immune cells. The submucosa comprises blood vessels, lymph nodes and particularly fat tissue. The inner circular musculature of the muscularis is strongly pronounced whereas the outer longitudinal musculature is practically only found in the taeniae.
The main task of the colon is the temporary storage and transport of the feces. Thereby it daily absorbs about 1 liter of water which leads to a thickening of the stool. Furthermore it absorbs sodium, potassium and chloride but can also secrete potassium into the lumen itself. The physiological intestinal flora is rich in anaerobic bacteria (approx. 1011/g) which live in symbiosis with the human body. They fulfill essential functions such as decomposing indigestible food ingredients (e.g. cellulose), producing vitamin K, promoting the intestinal peristalsis and supporting the immune system.